During World War II there was great debate in Cambodia as they remained under French control, as did their neighbors. However the Siamese were seeking independence so sided with Japan in order to gain their freedom from French domination. The Cambodians weren't as eager at first so stuck with the French, leading to invasions by the Siamese and Japanese, who took the land late in the war. However this takeover had a positive consequence when the Japanese granted the Cambodians freedom and allowed them to use their historic alphabet, which the French refused.
Despite this short lived freedom, after the Japanese lost World War II the French re-took control over the county and the region despite Cambodian protests. From this time Cambodia remained fairly neutral in international affairs, however this ended in the late 1960s and early 1970s when war broke out in neighboring Vietnam. The United States and South Vietnam began bombing North Vietnamese stronghold both in Vietnam as well as inside Cambodia's border, something they probably had permission to do from Prince Sihanouk, although he publicly denounced these bombings. Despite this, he refused to allow the U.S. to use his air space or air fields for the war, creating a divide between the two countries.
Through the 1960s and 1970s there was also a growing separation between people's political opinions as a growing communist regime with an inclination for violence began a rise to power. The group of communists, led by Saloth Sar (later known as Pol Pot) began to become very polarizing, while also gaining greater and greater power; they were known as the Khmer Rouge (which means "Red Khmer" in French, with red being the color of communism).
In 1970 Prince Sihanouk was overthrown by a military coup and the new leadership immediately spoke out against the growing communists and allied with the United States. They also formally established the Khmer Republic. However this union led to the United States entering Cambodia to fight the Khmer Rouge as well as North Vietnamese soldiers seeking refuge in Cambodia and this led to pressure from the people on the new government.
These arguments from the people in coordination with arguments at the top of the government led to a slow takeover by the communists in the early 1970s. This led to greater support of the Khmer Rouge and the eventual collapse of the Khmer Republic in 1975.
The Khmer Rouge was one of the most violent and aggressive government in recent history and after taking power they sent the people from the cities to the fields to work, religion was suppressed, all industries were taken under state control, and the banking system was abolished since all were equal and would what they needed from the government. Needless to say, many of these tactics didn't work in the long run.
The Khmer Rouge immediately destroyed relations with their neighbors as Vietnam cut off ties and even attacked them. They then turned to China for support, a move partially done to counteract the Soviet's support of Vietnam.
Cambodia immediately fell into chaos and poverty as the country's new "farmers" from the cities didn't know anything about agriculture so production fell and thousands of people starved to death. Additionally, the government killed thousands of people en masse in what are now known as "killing fields." The government rounded up people who wore glasses, spoke a different language, or had a well-paying job before the takeover and killed them in these fields, often times beating them to death in order to save bullets. They even killed some Khmer Rouge loyalists who they claimed didn't find enough counter-revolutionaries to kill. How many people actually died is unknown, estimates range from hundreds of thousands to three million.
The Khmer Rouge ended in 1979 when the Vietnamese communists took over the country, however not before a civil war. This led to new communist leadership, which fortunately ended most of the violence. They retained control through the 1980s as the country settled down, although violence and battles never ended.
In 1992, the United Nations entered Cambodia to enforce a cease fire and moved the hundreds of thousands of Cambodians who had fled to neighboring countries or were simple displaced in their own country. This led to 1993 elections in which the people choose a new path forward, although numerous communist rebel groups prevented many from attending the polling stations. This led to a constitutional monarchy, in which Prince Sihanouk became king.
Since 1993 the country has been very stable, although the country remains littered with landmines, a favorite of the Khmer Rouge government. Tourism has also increased, most notably to the world famous site of Angkor. In 2008 trials began against senior Khmer Rouge leaders.